Reciprocating compressors generally utilize the rectilinear motion of a piston within a cylinder to compress a working fluid. The rectilinear motion may be provided by the connection of the piston to a crankshaft via a connecting rod. In forming the connection, the crankshaft may be connected to the connecting rod via a crankpin. However, as arranged, sideways forces from the connecting rod may be transmitted directly to the piston, as the crankpin moves from side to side with the rotary motion of the crankshaft.
In large reciprocating compressors, these constant sideways forces result in an increase in overall friction and an undesirable amount of wear on the piston and cylinder. Accordingly, in an effort to decouple the transverse movement of the crankpin from the axial movement of the piston, those of skill in the art typically employ a crosshead-piston rod assembly. In such an assembly, a crosshead links a piston rod (a component of the piston) with the connecting rod. Thus, the sideways forces transmitted by the connecting rod are absorbed by the crosshead, which allows the piston rod to move along a longitudinal axis thereof with negligible transverse load.
To carry out the foregoing, the crosshead may be linked or coupled to the piston rod in various manners. For example, the crosshead may include a flange attached to an axial end of a main body of the crosshead. A neck of the piston rod may be inserted into a bore defined by an inner surface of the flange, such that a threaded end of the piston rod may extend from the bore and a shoulder of the piston rod may contact the surface of the flange. A piston rod nut may be threaded onto the threaded end of the piston rod, thus securing the flange to the piston rod. The flange may then be bolted to the main body of the crosshead, thus coupling the crosshead and the piston rod.
The piston rod neck may form one or more lands extending radially outward and configured to maintain alignment between the piston rod and the inner surface of the flange. During normal operation of the piston, axial and radial loads may cause a concentration of stress in the zone between the piston rod neck and shoulder and may also cause contact between the lands and the inner surface of the flange, thereby causing fretting and weakening of the piston rod. Accordingly, a proposed solution to the fretting has been the coating of the lands with a fretting resistant coating; however, certain drawbacks have been associated with this proposed solution. For instance, the application of the fretting resistant coating may result in increased expense and production time, as the coating typically must be applied at a third party facility after production of the piston rod, thereby leading to longer lead times for customers and greater potential for damage to the piston rod during shipping to and from the third party facility.
What is needed, therefore, is a facile and inexpensive manner of aligning and maintaining alignment of the piston rod with the flange in a crosshead-piston rod assembly while substantially reducing or eliminating fretting at the interface of the piston rod and flange.